Step response of a second order system is
\(s\left( t \right) = 1 - \frac{{{e^{ - 1.5t}}}}{{\sqrt {0.91} }}\sin \left[ {{\omega _n}\sqrt {0.91} t + 72.5^\circ } \right]\)
Damping frequency of the system is __________ rad/sec
Enter numerical value using the virtual keypad. Round off where necessary.